System and method for regulating exhaust emissions

ABSTRACT

An exhaust gas treatment system for an internal combustion engine includes an exhaust gas pathway configured to receive exhaust gas from the internal combustion engine, a first ammonia injector configured to inject ammonia into the exhaust gas pathway at a first rate, and a first treatment element positioned downstream of the first ammonia injector. A second ammonia injector is positioned downstream of the first treatment element. The second ammonia injector is configured to inject ammonia into the exhaust gas pathway at a second rate. A controller is configured to estimate an amount of particulate present in the exhaust gas and adjust at least one of the first rate or the second rate based on the estimate.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of co-pending U.S. patent applicationSer. No. 14/870,039, filed on Sep. 30, 2015, the entire content of whichis incorporated herein by reference.

BACKGROUND

The present disclosure relates to exhaust systems and more particularlyto diesel exhaust treatment systems.

SUMMARY

Diesel emissions include two primary components that are subject toemissions regulations: particulate matter and nitrogen oxides (NO_(x)).A variety of exhaust treatment devices have been developed to reducethese emission components. For example, a diesel particulate filter(DPF) can be used to trap diesel particulate matter and oxidize soot,and a selective catalytic reduction (SCR) element can be used to convertthe NO_(x) present in exhaust gas into other compounds, such as nitrogenand carbon dioxide. Typically, diesel exhaust fluid (DEF) is injectedupstream of the SCR element to provide ammonia, which acts as a reducingagent and reacts with the NO_(x) in the presence of the SCR catalyst.

A selective catalytic reduction on filter (SCR+F) element combines SCRand DPF functionality such that NO_(x) reduction and particulate matterfiltration and oxidation can occur in a single element. This can providea variety of advantages, including reduced size and cost. Sootoxidation, however, typically requires the presence of nitrogen dioxide(NO₂). Therefore, a tradeoff exists between NO_(x) reduction and sootoxidation when an SCR+F element is used.

In one embodiment, an exhaust gas treatment system for an internalcombustion engine includes an exhaust gas pathway configured to receiveexhaust gas from the internal combustion engine, a first ammoniainjector configured to inject ammonia into the exhaust gas pathway at afirst rate, and a first treatment element positioned downstream of thefirst ammonia injector. A second ammonia injector is positioneddownstream of the first treatment element. The second ammonia injectoris configured to inject ammonia into the exhaust gas pathway at a secondrate. A controller is configured to estimate an amount of particulatepresent in the exhaust gas and adjust at least one of the first rate orthe second rate based on the estimate.

In another embodiment, a method of treating exhaust gas from an internalcombustion engine as the exhaust gas passes through an exhaust gaspathway includes injecting ammonia, at a first rate, into the exhaustgas pathway at a first location. The method also includes treating theexhaust gas with a first treatment element positioned downstream of thefirst location, estimating an amount of particulate present in theexhaust gas, and adjusting the first rate based on the estimated amountof particulate.

In another embodiment, a method of treating exhaust gas from an internalcombustion engine as the exhaust gas passes through an exhaust gaspathway includes injecting ammonia, at a first rate, into the exhaustgas pathway at a first location. The method also includes convertingnitrogen oxides (NO_(x)) from the exhaust gas in a first treatmentelement positioned downstream of the first location, injecting ammonia,at a second rate greater than the first rate, into the exhaust gaspathway at a second location downstream of the first treatment element,and converting NO_(x) from the exhaust gas in a second treatment elementpositioned downstream of the second location.

Other features and aspects of the disclosure will become apparent byconsideration of the following detailed description and accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side view of a vehicle in which the disclosed system andmethod for regulating exhaust emissions may be implemented.

FIG. 2 is a schematic diagram of an exhaust gas treatment systemaccording to one embodiment.

FIG. 3 is a schematic diagram of a portion of an exhaust gas treatmentsystem according to another embodiment.

FIG. 4 is a cross-sectional view of a portion of the exhaust gastreatment system of FIG. 2, illustrating a flow-affecting featureaccording to one embodiment.

FIG. 5 is a cross-sectional view of a portion of the exhaust gastreatment system of FIG. 2, illustrating a flow-affecting featureaccording to another embodiment.

FIG. 6 is a block diagram of an electronic control unit of the exhaustgas treatment system of FIG. 2.

FIG. 7 is a flow diagram of operation of the exhaust gas treatmentsystem of FIG. 2.

FIG. 8 is a schematic diagram of an exhaust gas treatment systemaccording to another embodiment.

Before any embodiments are explained in detail, it is to be understoodthat the disclosure is not limited in its application to the details ofconstruction and the arrangement of components set forth in thefollowing description or illustrated in the following drawings. Thedisclosure is capable of supporting other embodiments and of beingpracticed or of being carried out in various ways. Also, it is to beunderstood that the phraseology and terminology used herein is for thepurpose of description and should not be regarded as limiting.

DETAILED DESCRIPTION

FIG. 1 illustrates an exemplary vehicle 10 including a diesel-poweredinternal combustion engine 14 and an exhaust gas treatment system 100according to one embodiment. The illustrated vehicle 10 is a utilitytractor, but the exhaust gas treatment system 100 is not so limited inapplication and can be used in conjunction with any diesel-poweredinternal combustion engine. For example, the exhaust gas treatmentsystem 100 can be used in other work vehicles, passenger vehicles, orother equipment powered by a diesel engine (e.g., generators,compressors, pumps, and the like).

With reference to FIG. 2, the exhaust gas treatment system 100 includesan exhaust pathway 104 (e.g., an exhaust pipe) having an inlet orupstream side 108 and an outlet or downstream side 112. A preliminarytreatment element 116, a first treatment element 120, and a secondtreatment element 124 are located in series along the exhaust pathway104, between the inlet 108 and the outlet 112. The numeric designations“first,” “second,” etc. are used herein for convenience and should notbe regarded as defining order, quantity, or relative position.

In the illustrated embodiment, an electronic control unit (ECU 123) isprovided to actively control various aspects of the operation of theexhaust gas treatment system 100. A sensor 125, which is a pressure drop(ΔP) sensor in the illustrated embodiment, is disposed proximate thefirst treatment element 120. The sensor 125 is communicatively coupledto the ECU 123 to provide feedback to the ECU 123 indicative of theperformance of the exhaust gas treatment system 100. In someembodiments, one or more additional sensors may be provided to monitorvarious other parameters of the exhaust gas treatment system 100. Thesesensors may monitor, for example, NO_(x) concentrations, ammoniaconcentrations, temperature, exhaust flow rate, and/or ash loading atone or more points along the exhaust pathway 104 and provide feedback tothe ECU 123 indicative of the performance of the exhaust gas treatmentsystem 100. In other embodiments, the exhaust gas treatment system 100may not be actively controlled, and the sensor 125 and/or the ECU 123may be omitted.

A first transition pipe 126 a interconnects the preliminary and firsttreatment elements 116, 120, and a second transition pipe 126 binterconnects the first and second treatment elements 120, 124. In theillustrated embodiment, the transition pipes 126 a, 126 b define anouter diameter that is smaller than an outer diameter of the treatmentelements 116, 120, 124. In an alternate embodiment illustrated in FIG.3, the treatment elements 116, 120, 124 are interconnected by transitionpipes 126 c, 126 d. The transition pipes 126 c, 126 d define an outerdiameter that is substantially the same as the outer diameter of thetreatment elements 116, 120, 124. As such, the treatment elements 116,120, 124 and the transition pipes 126 c, 126 d may collectively define acylindrical exhaust gas treatment unit 127. In the illustratedembodiment, the transition pipes 126 c, 126 d each define an overalllength that is less than their respective outer diameters. In someembodiments, the transition pipes 126 c, 126 d each define an overalllength between about 30% and about 70% of their respective outerdiameters.

The illustrated preliminary treatment element 116 is a diesel oxidationcatalyst (DOC) element and includes, for example, a honeycomb supportcoated with a catalytic material, such as a platinum group metal. Thepreliminary treatment element 116 may be used to reduce some particulatematter, carbon monoxide, and hydrocarbons from exhaust passing throughthe DOC element. Alternatively, the preliminary treatment element 116may include a different exhaust treatment configuration. In someembodiments of the exhaust treatment system 100, the preliminarytreatment element 116 may be omitted. In other embodiments, thepreliminary treatment element 116 may be included as a portion of adifferent treatment element (e.g., the first treatment element 120).

The first treatment element 120 in the illustrated embodiment is acombined selective catalytic reduction and diesel particulate filter(SCR+F) element and includes a catalytic washcoat on a porous filtersubstrate. The washcoat may include one or more base metal oxides, forexample, such as Al₂O₃, SiO₂, TiO₂, CeO₂, ZrO₂, V₂O₅, La₂O₃.Alternatively or additionally, the washcoat may include one or morezeolites. The first treatment element 120 may be used to captureparticulate matter, oxidize soot, and reduce NO_(x) from exhaust gaspassing through the first treatment element 120.

The second treatment element 124 in the illustrated embodiment includesa selective catalytic reduction (SCR) portion 128 and an ammoniaoxidation catalyst (AOC) portion 132. The SCR portion 128 may include,for example, a catalytic washcoat on a monolithic support material, suchas ceramic. The SCR portion 128 and the AOC portion 132 are positionedin series, with the AOC portion 132 located downstream of the SCRportion 128. The SCR portion 128 may be used to reduce NO_(x) fromexhaust gas passing through the SCR portion 128. The AOC portion 132 maybe used to convert excess ammonia leaving the SCR portion 128 tonitrogen and water. In some embodiments, the AOC portion 132 may beomitted. Alternatively, the AOC portion 132 may be provided as aseparate treatment element positioned downstream of the second treatmentelement 124.

With reference to FIG. 2, the exhaust gas treatment system 100 alsoincludes an ammonia source 136, which includes a diesel exhaust fluid(DEF) supply 140 and an ammonia producing unit 144 in the illustratedembodiment. The DEF supply 140 is in fluid communication with theammonia producing unit 144 to supply DEF (e.g., a urea solution) to theammonia producing unit 144, which converts the DEF to ammonia gas (e.g.,via thermolysis and hydrolysis). In some embodiments, a pump (not shown)is provided to move DEF from the DEF supply 140 to the ammonia producingunit 144. The pump may be variably controlled to vary the amount of DEFsupplied to the ammonia producing unit 144, and thus vary the amount ofammonia output by the ammonia producing unit 144. In other embodiments,the DEF may move from the DEF supply 140 to the ammonia producing unit144 under the influence of gravity. In such embodiments, one or morevalves (not shown) may be provided between the DEF supply 140 and theammonia producing unit 144 to vary the flow of DEF to the ammoniaproducing unit 144. In some embodiments, the DEF supply 140 may beomitted, and the ammonia producing unit 144 may include an ammoniasupply, such as a pressurized ammonia storage tank.

The exhaust gas treatment system 100 further includes a first injector148 and a second injector 152 in fluid communication with the ammoniaproducing unit 144. The first injector 148 and the second injector 152can be directly fluidly coupled to the ammonia producing unit 144 (e.g.,by independent conduits), or the first injector 148 and the secondinjector 152 can be fluidly coupled to a branch line, manifold, or otherstructure that receives ammonia from the ammonia producing unit 144. Thefirst injector 148 is positioned to introduce ammonia into the firsttransition pipe 126 a, downstream of the preliminary treatment element116 and upstream of the first treatment element 120 (i.e. between thepreliminary and first treatment elements 116, 120). The second injector152 is positioned to introduce ammonia into the second transition pipe126 b, downstream of the first treatment element 120 and upstream of thesecond treatment element 124 (i.e. between the first and secondtreatment elements 120, 124).

Referring to FIG. 4, the exhaust gas treatment system 100 may furtherinclude a flow affecting feature 156 in the exhaust pathway 104,positioned upstream of the first injector 148. In the illustratedembodiment, the flow affecting feature 156 is positioned within thefirst transition pipe 126 a. The flow affecting feature 156 can be, forexample, one or more fins, vanes, projections, or other suitable meansto impart turbulence or swirling into the exhaust flow proximate thefirst injector 148. With reference to FIG. 5, in another embodiment, theflow affecting feature 156 may be positioned downstream of the firstinjector 148. Although not illustrated in FIG. 4 or FIG. 5, another flowaffecting feature may be positioned in the second transition pipe 126 bupstream or downstream of the second injector 152.

With continued reference to FIGS. 4 and 5, the first injector 148 mayinclude an elongated portion 160 that extends into the exhaust pathway104. A plurality of openings 164 in the elongated portion 160 allowsammonia to be expelled from the injector 148 at multiple locations inthe exhaust pathway 104. Although not illustrated in FIG. 3, the secondinjector 152 may have a similar configuration.

With reference to FIG. 2, a first valve 168 is disposed between theammonia producing unit 144 and the first injector 148, and a secondvalve 172 is disposed between the ammonia producing unit 144 and thesecond injector 152. In some embodiments, the first and second valves168, 172 can be incorporated into the respective injectors 148, 152 orinto the ammonia producing unit 144. In other embodiments, the exhausttreatment system 100 may include only one valve (i.e., the first valve168 or the second valve 172). Alternatively, in some embodiments, thevalves 168, 172 may be omitted.

Each of the illustrated valves 168, 172 is movable between a closedposition in which the flow of ammonia through the valve 168, 172 issubstantially inhibited, and an open position in which the flow ofammonia through the valve 168, 172 is substantially uninhibited. In someembodiments, one or both of the valves 168, 172 can also be actuated toat least one intermediate position, between the closed and openpositions, in which the flow of ammonia through the valve 168, 172 ispartially restricted. In some embodiments, one or both of the valves168, 172 can be modulated to vary the flow rate of ammonia. In theillustrated embodiment, the valves 168, 172 are controlled by the ECU123.

FIG. 6 illustrates an example of the ECU 123 for control of the exhaustgas treatment system 100. The ECU 123 includes a plurality of electricaland electronic components that provide power, operational control, andprotection to the components and modules within the ECU 123. Inparticular, the ECU 123 includes, among other things, an electronicprocessor 180 (e.g., a programmable microprocessor, microcontroller, orsimilar device), non-transitory, machine-readable memory 184, and aninput/output interface 188. The electronic processor 180 iscommunicatively coupled to the memory 184 and configured to retrievefrom memory 184 and execute, among other things, instructions related tothe control processes and methods described herein. In otherembodiments, the ECU 123 includes additional, fewer, or differentcomponents. In the illustrated embodiment, the ECU 123 iscommunicatively coupled to the sensor 125, the DEF supply 140, the firstvalve 168, and the second valve 172. The ECU 123 may also be configuredto communicate with external systems including, for example, enginecontrols and/or operator controls.

In operation, untreated exhaust from the internal combustion engine 14(FIG. 1) is directed into the exhaust pathway 100 at the inlet 104 (FIG.2). The exhaust then flows through the preliminary treatment (DOC)element 116, which reduces some particulate matter, carbon monoxide, andhydrocarbons from the exhaust. Ammonia is introduced into thepartially-treated exhaust downstream of the preliminary treatmentelement 116 via the first injector 148. Because the first injector 148includes multiple holes 164, the ammonia is more uniformly dispersedinto the exhaust stream (FIG. 4). In addition, turbulence or swirlingmovement imparted by the flow affecting feature 156 enhances mixing tocreate a relatively homogeneous mixture of exhaust and ammonia within arelatively short distance from the injector 148. Thus, the distancebetween the preliminary and first elements 116, 120 can be minimized.

The ammonia and exhaust mixture then enters the first treatment (SCR+F)element 120 (FIG. 2). The ammonia reacts with NO_(x) in the presence ofthe catalyst to form nitrogen and water, while soot is captured andoxidized on the porous filter substrate. When the exhaust exits thefirst treatment element 120, additional ammonia is introduced via thesecond injector 152. Like the first injector 148, the second injector152 preferably includes a plurality of holes (not shown), to moreuniformly disperse the ammonia into the exhaust stream, and anadditional flow affecting feature (not shown) is preferably providedproximate the second injector 152 to further enhance mixing. Thus, thedistance between the first and second treatment elements 120, 124 can beminimized.

The ammonia and exhaust mixture then enters the second treatment element124, where the ammonia reacts with any remaining NO_(x) in the SCRportion 128, and any unreacted ammonia is subsequently oxidized in theAOC portion 132. The treated exhaust then exits the exhaust gastreatment system 100 through the outlet 108.

The amount of NO_(x) converted in the first and second treatmentelements 120, 124 is dependent upon the amount of ammonia injected. Inthe illustrated embodiment, the total amount of ammonia injected iscontrolled by the rate at which DEF is supplied to the ammonia producingunit 144. DEF flows from the DEF supply 140 to the ammonia producingunit 144 at a rate that may be controlled by the ECU 123, and theammonia producing unit 144 produces ammonia gas from the DEF. Theammonia gas flows from the ammonia producing unit 144, through thevalves 168, 172, and to the injectors 148, 152, which inject the ammoniainto the exhaust pathway 104.

Because proper soot oxidation in the first treatment element 120requires the presence of NO_(x) (specifically, NO₂), the amount ofammonia flowing through the first injector 148 is limited so that someof the NO_(x) remains unreacted through the first treatment element 120.In the illustrated embodiment, ammonia flows through the first injector148 at a first rate, and ammonia flows through the second injector 152at a second rate that is greater than the first rate. The ECU 123 mayadjust the flow of ammonia into the exhaust pathway 104 by modulatingthe first valve 168, the second valve 172, and/or the rate at which DEFflows to the ammonia producing unit 144.

With reference to FIG. 7, the ECU 123 may periodically or continuouslyread a value or signal from the sensor 125 at block 192 (via theinput/output interface 188), which is indicative of the pressure dropacross the first treatment element 120. The ECU 123 uses the sensedvalue to determine whether the first treatment element 120 is properlyoxidizing soot. The pressure drop may be correlated with the amount ofsoot or particulate present in the first treatment element 120. Forexample, as the amount of particulate in the first treatment element 120increases, the pressure drop increases, and as the amount of particulatein the first treatment element 120 decreases, the pressure dropdecreases. From this correlation, the ECU 123 can determine an estimatedparticulate parameter at block 194, which may correspond with anestimated amount of particulate in the first treatment element 120. Insome embodiments, the ECU 123 may periodically or continuously read avalue or signal from one or more additional or alternative sensors(e.g., NO_(x) sensors, ammonia sensors, temperature sensors, ash loadingsensors, exhaust flow rate sensors, etc.). The values or signals fromthese sensors may be factored into the ECU's determination of theestimated particulate parameter.

At block 196, the ECU 123 may then periodically or continuously comparethe estimated particulate parameter with a threshold value or valuerange, stored in memory 184, which is representative of a target sootoxidation performance level. If the estimated particulate parameter isgreater than the threshold value (i.e., if the level of particulate inthe first treatment element 120 is high), the ECU 123 decreases the flowof ammonia through the first injector 148 at block 200 by restrictingflow through the first valve 168. Accordingly, the amount of NO_(x)available for soot oxidation will increase. Optionally, the ECU 123 maythen increase the flow of ammonia through the second injector 152 atblock 202 by opening the second valve 172. If the estimated particulateparameter is less than the threshold value (i.e., if the level ofparticulate in the first treatment element 120 is low), the ECU 123 canincrease the flow of ammonia through the first injector 148 at block 204by opening the first valve 168. Accordingly, the amount of NO_(x) willbe reduced. Optionally, the ECU 123 may then decrease the flow ofammonia through the second injector 152 at block 206 by restricting flowthrough the second valve 172. In embodiments where either the firstvalve 168 or the second valve 172 is omitted, the first and secondinjectors 148, 152 are in fluid communication such that an increase inflow through the first injector 148 results in a proportional decreasein flow through the second injector 152, and vice versa.

FIG. 8 illustrates an exhaust gas treatment system 300 according toanother embodiment. The exhaust gas treatment system 300 is similar tothe exhaust gas treatment system 100 described above with reference toFIGS. 1-7. Accordingly, like features are given identical referencenumbers, and only differences between the exhaust gas treatment system300 and the exhaust gas treatment system 100 are described in detail.

The exhaust gas treatment system 300 includes a first ammonia passageway304 extending between the ammonia producing unit 144 and the firstammonia injector 148 and a second ammonia passageway 308 extendingbetween the ammonia producing unit 144 and the second ammonia injector152. The first ammonia passageway 304 defines a first diameter, and thesecond ammonia passageway 308 defines a second diameter that is greaterthan the first diameter. In some embodiments, the passageways 304, 308may have the same outer diameter but different inner diameters. In otherembodiments, the passageways 304, 308 may have the same outer and innerdiameters, and the first passageway 304 may include a restriction thatreduces the effective inner diameter of the first passageway 304.

In operation, ammonia flows from the ammonia producing unit 144 to theinjectors 148, 152 via the respective passageways 304, 308. Because thefirst passageway 304 is relatively restricted compared to the secondpassageway 308, ammonia flows through the first injector 148 at a firstrate and through the second injector 152 at a second rate greater thanthe first rate. The passageways 304, 308 are sized to provide someNO_(x) reduction in the first treatment element 120 and a greater amountof NO_(x) reduction in the SCR portion 128 of the second treatmentelement 124. This configuration may preserve enough NO_(x) in the firsttreatment element 120 for effective soot oxidation.

Various features of the disclosure are set forth in the followingclaims.

What is claimed is:
 1. An exhaust gas treatment system for an internalcombustion engine, the system comprising: an exhaust gas pathwayconfigured to receive exhaust gas from the internal combustion engine; afirst ammonia injector configured to inject ammonia into the exhaust gaspathway at a first rate; a first treatment element positioned downstreamof the first ammonia injector; a second ammonia injector positioneddownstream of the first treatment element, the second ammonia injectorconfigured to inject ammonia into the exhaust gas pathway at a secondrate; and a controller configured to estimate an amount of particulatepresent in the exhaust gas and adjust at least one of the first rate orthe second rate based on the estimate.
 2. The exhaust gas treatmentsystem of claim 1, further comprising a sensor configured to sense atleast one parameter selected from a group consisting of temperature,pressure, flow rate, ash loading, NO_(x) concentration, and ammoniaconcentration.
 3. The exhaust gas treatment system of claim 2, whereinthe controller is configured to estimate the amount of particulatepresent in the exhaust gas based on the sensed parameter.
 4. The exhaustgas treatment system of claim 1, further comprising a second treatmentelement positioned downstream of the second ammonia injector.
 5. Theexhaust gas treatment system of claim 1, wherein the second rate isgreater than the first rate.
 6. The exhaust gas treatment system ofclaim 1, further comprising a first valve disposed upstream of the firstammonia injector, and a second valve disposed upstream of the secondammonia injector.
 7. The exhaust gas treatment system of claim 6,wherein the controller is configured to modulate the first valve toadjust the first rate and to modulate the second valve to adjust thesecond rate.
 8. The exhaust gas treatment system of claim 1, furthercomprising a flow affecting feature positioned in the exhaust gaspathway upstream of the first ammonia injector, the flow affectingfeature configured to create turbulence within the exhaust gas pathwayto facilitate mixing the exhaust gas with the ammonia from the firstammonia injector.
 9. The exhaust gas treatment system of claim 1,wherein the first ammonia injector includes a plurality of openings,each of the openings configured to introduce the ammonia into theexhaust gas pathway at a different position.
 10. A method of treatingexhaust gas from an internal combustion engine as the exhaust gas passesthrough an exhaust gas pathway, the method comprising: injectingammonia, at a first rate, into the exhaust gas pathway at a firstlocation; treating the exhaust gas with a first treatment elementpositioned downstream of the first location; estimating an amount ofparticulate present in the exhaust gas; and adjusting the first ratebased on the estimated amount of particulate.
 11. The method of claim10, wherein estimating includes monitoring a pressure drop across thefirst treatment element.
 12. The method of claim 10, further comprisinginjecting ammonia, at a second rate different than the first rate, intothe exhaust gas pathway at a second location downstream of the firsttreatment element.
 13. The method of claim 10, further comprisingtreating the exhaust gas with a second treatment element positioneddownstream of the first treatment element.
 14. The method of claim 10,wherein treating the exhaust gas with the first treatment elementincludes converting nitrogen oxides (NO_(x)) from the exhaust gas.
 15. Amethod of treating exhaust gas from an internal combustion engine as theexhaust gas passes through an exhaust gas pathway, the methodcomprising: injecting ammonia, at a first rate, into the exhaust gaspathway at a first location; converting nitrogen oxides (NO_(x)) fromthe exhaust gas in a first treatment element positioned downstream ofthe first location; injecting ammonia, at a second rate greater than thefirst rate, into the exhaust gas pathway at a second location downstreamof the first treatment element; and converting NO_(x) from the exhaustgas in a second treatment element positioned downstream of the secondlocation.
 16. The method of claim 15, further comprising determining aparticulate parameter indicative of an amount of particulate present inthe exhaust gas; and adjusting at least one of the first rate or thesecond rate based on the particulate parameter.
 17. The method of claim16, wherein determining the particulate parameter includes correlating asensed parameter with an amount of particulate present in the exhaustgas.
 18. The method of claim 17, wherein the sensed parameter representsa pressure difference measured across the first treatment element. 19.The method of claim 15, wherein adjusting further includes modulating afirst valve to adjust the first rate and modulating a second valve toadjust the second rate.
 20. The method of claim 15, wherein injectingammonia at the first location includes passing the ammonia through afirst passageway defining a first diameter, and wherein injectingammonia at the second location includes passing the ammonia through asecond passageway defining a second diameter greater than the firstdiameter.